1. Field of the Invention
This invention relates generally to the field of polycrystalline diamond compact (PDC) drilling bits. More specifically, this invention relates to PDC bits which drill a hole through earth formations where the drilled hole has a larger diameter than the "pass-through " diameter of the drill bit.
2. Description of the Related Art
Drill bits which drill holes through earth formations where the hole has a larger diameter than the bit's pass-through diameter (the diameter of an opening through which the bit can freely pass) are known in the art. Early types of such bits included so-called "underreamers ", which were essentially a drill bit having an axially elongated body and extensible arms on the side of the body which reamed the wall of the hole after cutters on the end of the bit had drilled the earth formations. Mechanical difficulties with the extensible arms limited the usefulness of underreamers.
More recently, so-called "bi-centered " drill bits have been developed. A typical bi-centered drill bit includes a "pilot " section located at the end of the bit, and a "reaming" section which is typically located at some axial distance from the end of the bit (and consequently from the pilot section). One such bi-centered bit is described in U.S. Pat. No. 5,678,644 issued to Fielder, for example. Bi-centered bits drill a hole larger than their pass through diameters because the axis of rotation of the bit is displaced from the geometric center of the bit. This arrangement enables the reaming section to cut the wall of the hole at a greater radial distance from the rotational axis than is the radial distance of the reaming section from the geometric center of the bit. The pilot section of the typical bi-centered bit includes a number of PDC cutters attached to structures ("blades ") formed into or attached to the end of the bit. The reaming section is, as already explained, typically spaced axially away from the end of the bit, and is also located to one side of the bit. The reaming section also typically includes a number of PDC inserts on blades on the side of the bit body in the reaming section.
Limitations of the bi-centered bits known in the art include the pilot section being axially spaced apart from the reaming section by a substantial length. FIG. 1 shows a side view of one type of bi-center bit known in the art, which illustrates this aspect of prior art bi-center bits. The bi-center bit 101 includes a pilot section 106, which includes pilot blades 103 having PDC inserts 110 disposed thereon, and includes gauge pads 112 at the ends of the pilot blades 103 axially distant from the end of the bit 101. A reaming section 107 can include reaming blades 111 having PDC inserts 105 thereon and gauge pads 117 similar to those on the pilot section 106. In the bi-center bit 101 known in the art, the pilot section 106 and reaming section are typically separated by a substantial axial distance, which can include a spacer or the like such as shown at 102. Spacer 102 can be a separate element or an integral part of the bit structure but is referred to here as a "spacer " for convenience. As is conventional for drill bits, the bi-center bit 101 can include a threaded connector 104 machined into its body 114. The body 114 can include wrench flats 115 or the like for make up to a rotary power source such as a drill pipe or hydraulic motor.
An end view of the bit 101 in FIG. 1 is shown in FIG. 2. The blades 108A in the pilot section and the blades 111B in the reaming section are typically straight, meaning that the cutters 110 are disposed at substantially the same relative azimuthal position on each blade 108A, 111B. In some cases the blades 108A in the pilot section 106 may be disposed along the same azimuthal direction as the blades 111B in the reaming section 110.
Prior art bi-center bits are typically "force-balanced "; that is, the lateral force exerted by the reaming section 110 during drilling is balanced by a designed-in lateral counterforce exerted by the pilot section 106 while drilling is underway. However, the substantial axial separation between the pilot section 106 and the reaming section 110 results in a turning moment against the axis of rotation of the bit, because the force exerted by the reaming section 110 is only balanced by the counterforce (exerted by pilot section 106) at a different axial position. This turning moment can, among other things, make it difficult to control the drilling direction of the hole through the earth formations.
Still another limitation of prior art bi-centered bits is that the force balance is calculated by determining the net vector sum of forces on the reaming section 110, and designing the counterforce at the pilot section 106 to offset the net vector force on the reaming section without regard to the components of the net vector force originating from the individual PDC inserts. Some bi-center bits designed according to methods known in the art can have unforeseen large lateral forces, reducing directional control and drilling stability.